Method and device for playing optical discs and method for determining tracking quality

ABSTRACT

A method for reading data from a track ( 3 ) of a data carrier ( 2 ) comprises the following steps. During reading, a tracking quality parameter (Q) is determined, relating to a track portion ( 3 Px) of the data carrier from which the data is being read. The tracking quality parameter is stored in a tracking performance memory ( 30 ) in relation to the corresponding track portion ( 3 Px). If a jump back to a previously read track portion appears to be necessary, said tracking performance memory ( 30 ) is consulted to determine a target track portion ( 3 P(x-n)) having a sufficient value of the tracking quality parameter (Q); then, a jump back (JB) to the target track portion ( 3 P(x-n)) thus determined is performed.

The present invention relates in general to a method for reading datafrom a data carrier. Specifically, the present invention relates to amethod for reading data from an optical storage disc; the invention willhereinafter be explained for the example of a DVD-disc without it beingintended to restrict the scope of the invention.

As is commonly known, an optical storage disc comprises at least onetrack, either in the form of a continuous spiral or in the form ofmultiple concentric circles, of storage space where information may bestored in the form of a data pattern. For reading information from thedisc, an optical disc drive comprises, on the one hand, rotating meansfor receiving and rotating an optical disc, and on the other hand anoptical pickup which is mounted radially displaceable, for scanning thestorage track with a laser beam. Since the technology of optical discsin general, the way in which information can be stored in an opticaldisc, and the way in which optical data can be read from an opticaldisc, is commonly known, it is not necessary here to describe thistechnology in more detail.

During operation, the disc is rotated at a certain rotational speed,resulting in a read data stream having a certain data rate. Via a buffermemory, the disc drive provides this data stream as output stream, foran application such as a computer program running on a PC. The data rateof the output data stream should correspond to the data rate requestedby the application. By way of example of an application, a DVD videoplayback is mentioned. For playing a movie from DVD withoutinterruptions, the DVD playback application needs to receive a stream ofdata (real-time video and audio) at a certain output data rate.

The data rate requested by the application can vary quickly. Onepossibility for the disc drive to always provide the requested data rateis to change the rotational frequency quickly, following the requestsmade by the application. However, this is not desired as speed changesrequire much energy. Therefore, the disc drive comprises a data buffer.The data as read from disc by the disc drive is fed into said buffermemory; the application reads the data from the buffer memory, at avariable rate. Thus, the filling level of the buffer memory may varywith time.

Still, the disc drive may have variable disc rotation frequency, whereinthe speed changes are now mitigated by the buffer, but substantialenergy savings can only be achieved with unreasonably large buffers.Therefore, it is preferred that the disc drive has a fixed or slowlyvarying disc rotation frequency.

The rotation frequency of the disc may not be chosen too low: in thatcase, a buffer-underrun will occur, i.e. the average data output ratefrom the buffer is larger than the data input rate into the buffer, andthe buffer becomes empty. In the case of a video playback, this wouldresult in an interruption of the playback. To avoid this, the discrotation frequency is set at a relatively high level, guaranteeing adata rate higher than the maximum data rate as requested by theapplication. This specific rotational frequency is called “overspeed”.As a consequence, a buffer overrun will occur, i.e. the buffer becomesfull. This means that the disc drive has to jump back some tracks, inorder to read again the data that could not be stored in the buffer.

The overspeed factor (i.e. the ratio between read data rate andapplication read rate, i.e. the ratio between buffer input rate andbuffer output rate), cannot be chosen marginally larger than 1. If readerrors occur, the disc drive also has to jump back in order to retry toread the corresponding data; during this retry, the application shouldbe able to continue reading data from said buffer memory, and the bufferfilling level should be large enough to allow this. On the other hand,after a retry, the disc drive should bring back the buffer memory to itsfilling level as soon as possible. As a consequence of the relativelyhigh overspeed factor, the disc drive has to jump back relatively often,even under normal circumstances. In case of a bad disc, e.g. a heavilyscratched disc, many read errors occur, and the jumps back occur evenmore frequently.

A problem associated with jumps is that, after each jump, the disc drivehas to regain tracking and possibly focus. Even in the case of a gooddisc, this will take some time. In the case of a bad disc, e.g. aheavily scratched disc, regaining tracking and/or focus may prove to bevery difficult and take a long time, to such extent, that the targettrack is missed and a further retry is necessary, increasing the risk ofa buffer underrun.

A main objective of the present invention is to reduce these problems.

According to an important aspect of the present invention, a method forreading data from a track of a data carrier, comprises the steps of:

tracking the track while reading data from the track, using a trackingperformance memory which contains information relating to a trackingquality of track portions of the data carrier, to determine, insituations where tracking has to be restored, which track portion tojump to for restoring tracking.

The tracking performance memory can be a memory chip on board of anoptical disc drive, or any other memory which can also be locatedoutside the disc drive. For instance, the tracking performance memorycan be a database which is maintained on the internet. Alternatively,the tracking performance memory may be a memory located on a centralcomputer system in a network.

The tracking can be lost due to for instance an external shock. Alsoduring a jump from one track portion to another, tracking may be lostand has to be restored. When using the tracking performance memory, atrack portion can be selected which has a good tracking quality, i.e. atrack portion on which the tracking can be relatively easy restored.

In a further embodiment of the invention the method further comprisesthe steps of:

during reading, determining a tracking quality parameter relating to atrack portion of the data carrier from which the data is being read;

storing the tracking quality parameter in the tracking performancememory in relation to the corresponding track portion.

In this embodiment the tracking performance memory is maintained by thismethod by storing the tracking quality parameter in this memory.Alternatively, the memory is filled using an other method or apparatus.

In a still further embodiment the method further comprises the steps of:

determining if a jump back to a previously read track portion isnecessary;

if a jump back to a previously read track portion appears to benecessary, consulting said tracking performance memory to determine atarget track portion having a sufficient value of the tracking qualityparameter;

jumping back to the target track portion thus determined.

In this embodiment, a disc drive maintains a quality memory whichcontains information relating to the quality of track portions beingread. Later, when a jump back is considered necessary, the informationin this quality memory is consulted to determine a target location forthe jump. This target location is selected such that it lies in a “good”track portion, i.e. a track portion of which it is expected thattracking and/or focus will be regained relatively easily and thereforequickly.

According to another aspect of the invention a method of determining atracking quality of track portions of a data carrier, comprises thesteps of:

tracking the track;

during tracking, determining a tracking quality parameter relating to atrack portion of the data carrier;

storing the tracking quality parameter in a tracking performance memoryin relation to the corresponding track portion.

The tracking performance memory can be advantageously utilized in themethod for reading data according to the invention.

These and other aspects, features and advantages of the presentinvention will be further explained by the following description withreference to the drawings, in which same reference numerals indicatesame or similar parts, and in which:

FIG. 1 schematically shows relevant components of a disc drive;

FIG. 2 schematically illustrates data reading and buffering;

FIG. 3 schematically illustrates the operation of a tracking performancememory;

FIGS. 4A-C are graphs schematically showing exemplary content of thetracking performance memory;

FIG. 5 is a flow diagram illustrating a read procedure.

FIG. 1 schematically shows relevant components of a disc drive 1 forreading information from an optical disc 2, for instance a DVD disc. Forrotating the disc 2, the disc drive 1 comprises a motor 4 fixed to aframe (not shown for sake of simplicity), defining a rotation axis 5.The disc drive further comprises an optical pickup 6, which comprisesmeans for generating a laser beam 7, focusing the laser beam 7 on aninformation layer of the disc 2, receiving reflected laser light, andgenerating a read signal S_(R) which is received by a controller 10. Theoptical pickup 6 is mounted radially displaceable, and the disc drive 1comprises a radial actuator 8, which is controlled by the controller 10,for controlling the radial position of the pickup 6 such that the laserbeam 7 follows a track (not shown for sake of simplicity) of the disc 2or is jumped to a different track position.

Since optical disc drives are known per se, and the technology ofradially displaceable optical pickups for reading tracks of opticaldiscs is known per se, a more detailed explanation of the optical pickup6 and the controller 10 is omitted here.

The disc drive 1 further comprises a data buffer memory 20, having aninput 21 coupled to a first output 11 of the controller 10, and havingan output 22 coupled to an output 9 of the disc drive 1.

The operation of a read procedure of the disc drive 1 is explained withreference to FIG. 2, which illustrates a track 3 of the disc 2 as alongitudinal strip of data. The track 3 has a beginning 3A and an end3Z. Usually, the beginning 3A corresponds to the inner radius of thedisc while the end 3Z corresponds to the outer radius of the disc. Thepickup 6 is shown at a certain position with respect to the track 3. Byvirtue of the rotation of the disc 2, the pickup 6 scans the track 3,illustrated by arrow P1 in FIG. 2, and a data signal S_(R),corresponding to the data read from track 3, is generated and receivedby the controller 10.

In FIG. 2, the buffer memory 20 is illustrated as a longitudinal stripof memory locations 23. Reference numeral 20A indicates an empty portionof the buffer memory 20 where the memory locations do not containrelevant data, while reference numeral 20B indicates a filled portion ofthe buffer memory 20 where the memory locations are filled with relevantdata. The boundary between the empty memory portion 20A and the filledmemory portion 20B is indicated at 20C. The controller 10 stores newdata read from track 3 always at the first memory location of the emptymemory portion 20A, as indicated by arrow DATA_(IN), thus decreasing thesize of the empty memory portion 20A and increasing the size of thefilled memory portion 20B, in other words shifting the boundary 20C tothe left in FIG. 2. An application always reads data from the firstmemory location 23 a of the filled memory portion 20B, as indicated byarrow DATA_(OUT). After such read process, the contents of the memorylocations 23 of the filled memory portion 20B are shifted to thecorresponding neighboring memory locations 23 towards in a directiontowards the said first memory location 23 a of the filled memory portion20B, as indicated by arrows SHIFT. After such shift process, said firstmemory location 23 a of the filled memory portion 20B again contains thenext data to be read by the application. The shift process increases thesize of the empty memory portion 20A and decreases the size of thefilled memory portion 20B, in other words shifts the boundary 20C to theright in FIG. 2.

In operation, the position of the boundary 20C will vary, depending onthe input data rate (which is constant in the case of video playback)and the output data rate (which varies, depending on the actualcompression rate of the data stored on disc). In order to avoid abuffer-underrun (which means that the boundary 20C meets the firstmemory location 23 a of the filled memory portion 20B, so that theentire buffer memory 20 is empty), the average input data rate should behigher than the maximum output data rate, so that eventually abuffer-overrun occurs, which means that the boundary 20C meets the lastmemory location 23 z of the empty memory portion 20A, so that the entirebuffer memory 20 is full. If reading now continues, the read data cannotbe stored in the buffer memory 20, so that a playback error wouldresult; this is avoided by a jump back of the optical pickup.

In operation, it may also happen that a read error occurs, for instancedue to scratches on the disc surface. In that case, the correspondingdata cannot be stored in the buffer memory as well, and a playback errorwould occur also. Again, this is avoided by a jump back of the opticalpickup.

Such a jump back is indicated by the arrow JB in FIG. 2. The pickup 6 ismoved back to a location before the location where the error occurred,i.e. to a location closer to the beginning 3A; the location after jumpback is indicated in dotted lines in FIG. 2. From the new location afterjump back, the optical pickup 6 starts to read again (in the directionof said arrow P1), but it takes some time before the pickup has regainedtracking and/or focus. During this time, as well as during the jump, theoutput of data from buffer 20 to the application continues, resulting inthe decrease of the size of the filled memory portion 20B. Filling ofthe buffer is resumed when the optical pickup 6 arrives at the locationwhere the read error occurred, or where the buffer-overrun occurred.More precisely: filling of the buffer is resumed when the optical pickup6 arrives at the end of the last complete ECC block that was readcorrectly from disc.

In the state of the art, the jump back has a fixed length, or the targetarrival of the jump is selected in accordance with the contents of thedata on disc, for instance just before a V_(SYNC). A problem of suchprior art approach is that, depending on the quality of the disc, thereexists a chance that the jump may end in an area with bad trackingquality, for instance an area with a badly scratched surface, whichmakes it very difficult to regain tracking and/or focus.

According to a special feature of the present invention, this chance isreduced by assuring that the jump ends in an area with good trackingquality. To this end, the disc drive 1 further comprises a trackingperformance memory 30, and the controller 10, during playback,determines a value of a tracking quality parameter as a measure of thetracking performance of the disc, and stores this parameter in thetracking performance memory 30. This will be explained in more detailwith reference to FIG. 3, where the tracking performance memory 30 isillustrated as a longitudinal step of memory locations 33. This memory30 may be considered to be a shift memory, having a first or inputmemory location 33 a and a last memory location 33 z. In each writeaction by the controller 10, the controller writes information into theinput memory location 33 a while the contents of each memory location 33is shifted one memory location away from the input memory location 33 a,i.e. to the right in FIG. 3, as indicated by arrows SHIFT in FIG. 3; thecontents of the last memory location 33 z is thrown away, as indicatedby arrow EXIT.

Defining and measuring the tracking quality parameter, indicatedhereinafter simply as Q, can be done in various ways. For instance, Qcan be derived from the shape of an error signal, such as the errorsignals involved in beam focus control and track following control, aswill be known to persons skilled in this art. But other signals orcombinations of signals can also be used. In the following, for the sakeof exemplary discussion, it will be assumed that Q can take a valuebetween 0 and 10, wherein 10 indicates ideal tracking quality and 0indicated very bad tracking quality with many errors, but it is to beunderstood that this is not intended to restrict the invention.

Further, a predetermined tracking quality threshold Q_(T) is determined,which is a value of Q which is considered to be “good enough”. In otherwords, if the Q of a track portion has a value above Q_(T), this trackportion is available as jump target. In this example, in which Q canrange from 0 to 10, it will be assumed that Q_(T)=7.

In principle, Q can be determined in association with each data byte,but such high resolution is not necessary; in practice, it suffices if Qis determined in association with track portions having a predefinedlength. For instance, each 360° track may be subdivided into 64 trackportions of 5.625°, but another subdivision will also be possible. Avalue of Q is determined for each such track portion; therefore, in hissexample, after each revolution of the disc 2, 64 samples of Q arewritten into the tracking performance memory 30. Further, assume that itis desirable to be able to jump as far back as 32 tracks. Thiscorresponds to 32×64 track portions which are potential jump targets.Therefore, for this example, the tracking performance memory 30 needs tohave only 2048 memory locations.

In FIG. 2, track portions are generally indicated at reference numeral3P; individual track portions are distinguished by adding index 1, 2, 3,. . . i, . . . etc.

FIGS. 4A-C are graphs showing the content of the tracking performancememory 30 for different hypothetical discs, illustrating possibleexamples of the tracking quality of a disc.

In the case illustrated in FIG. 4A, the disc has very good trackingquality in all disc regions except a few exceptions: in such case, thereare only few restrictions to the locations where a jump might end.

In the case illustrated in FIG. 4B, the disc has bad tracking quality.Almost in all locations, Q is below Q_(T); only three areas have a Qabove Q_(T).

In the case illustrated in FIG. 4C, the disc has very bad trackingquality: Q is always below Q_(T).

In the following, a preferred example of a jump strategy will bedescribed, for determining a jump target location.

First, at the moment of encountering the error, the maximum jump lengthis determined, which depends on the position of the boundary 20C, i.e.the size of the filled memory portion 20B. If the buffer memory 20 isalmost empty, the jump can only be relatively short, whereas the jumpcan be relatively long if the buffer memory 20 is almost full. Afterall, the length of the jump determines how much time is needed beforethe error location is reached again, and this should correspond to theplaytime left in the buffer memory. The maximum jump length correspondsto a maximum jump location in the tracking performance memory 30,indicated as maximum boundary 41. The portion of the trackingperformance memory 30 between said maximum boundary 41 and the firstmemory location 33A will be indicated as available jump area 42.

It is possible that the jump strategy determines the memory locationwith highest Q within the available jump area 42. In the examples ofFIGS. 4A-C, this memory location is indicated as maximum trackingquality location 43. However, such is not necessary in the casesillustrated in FIGS. 4A and 4B, when the disc has a sufficient number oftrack portions having a Q above Q_(T). In that case, a strategypreferred in view of its simplicity results in a jump to the closesttrack portion 3P(x-n), i.e. having the smallest value of n, for whichthe corresponding memory location contains a Q above Q_(T), indicated at44 in FIG. 4A and 4B. In the above, 3Px indicates the current trackportion, and n indicates the jump length in track portions.

In a very bad case, as illustrated in FIG. 4C, where Q is always belowQ_(T), the jump strategy determines the “best” location as being the“least bad” location, i.e. the memory location with highest Q within theavailable jump area 42. A jump ending in track portion corresponding tothis “best” location has the highest probability of regaining trackingand focus within a reasonable time.

In the example of FIG. 4C, the highest Q occurs at two memory locations,indicated at 43A and 43B. It is possible to decide to jump to the trackportion 3P(x-n_(A)) corresponding to the closest “best” location 43A,i.e. having the smallest value of n. In that case, the jump isrelatively short. If regaining tracking and/or focus fails, the timeleft for a retry is as large as possible.

On the other hand, it is also possible to decide to jump to the trackportion 3P(x-n_(B)) corresponding to the most remote “best” location43B, i.e. having the largest value of n. In that case, the jump isrelatively long, giving the disc drive more time to regain tracking andfocus before reaching the current location 3Px again. It is noted that,after having landed on the track portion 3P(x-n_(B)) corresponding tothe most remote “best” location 43B, if the disc drive is notimmediately successful with regaining tracking and focus, the disc drivecomes to pass the track portion 3P(x-n_(A)) corresponding to the closest“best” location 43A, and thus has a second opportunity for regainingtracking and focus at the target track location which would have beendecided by the jump strategy mentioned earlier.

The invention will now be described in more detail with reference toFIG. 5, which is a flow diagram illustrating an embodiment of a readprocedure 500 in accordance with the invention.

The procedure starts [step 501] when the start address of an ECC blockis reached; this address is stored in an ECC start memory [step 502].

The track of the disc is scanned, and the read signal is decoded to readthe bytes contained therein [step 511]. In step 512, it is determinedwhether an error occurs, such as a read error or a buffer overrun. If noerror occurs, the byte is written [step 513] into the data buffer memory20, as described above, so that the filled buffer portion 20B grows.Further, the tracking quality is determined [step 514]. This procedureis repeated, byte-by-byte, until a new track portion is reached [step521] or a new ECC block is reached [step 531].

In step 521, it is determined whether a new track portion is reached. Ifso, then a value for Q, valid for the entire track portion 3Px justfinished, is calculated [step 522] on the basis of the tracking qualityas experienced during reading; for instance, the tracking quality valueQ accorded to a track portion may be chosen equal to the lowest trackingquality value experienced during reading of this track portion. Thecalculated Q is stored in the tracking performance memory 30 [step 523]as described earlier.

In step 531, it is determined whether a new ECC block is reached. If so,then the address of the new ECC block is stored in said ECC start memory[step 502]. Thus, this memory always contains the start address of theECC block being read.

If, in step 512, it appears that an error occurs, reading is stopped,and processing jumps to a jump strategy procedure 540. The size of thefilled portion 20B of the data memory 20 is determined [step 541], andbased on this size a maximum jump length is calculated [step 542], whichtranslates into a last memory location in the tracking performancememory 30. Then, the tracking performance memory 30 is analyzed in ananalyzing procedure 550.

A variable Q_(MAX), which indicates the maximum value of the trackingquality parameters Q stored in the relevant portion of the trackingperformance memory 30, is reset to have value 0 [step 551]. A firstmemory location in the tracking performance memory 30 is determined[step 552]; this first memory location corresponds to a minimum jumplength, which takes into account the address stored in said ECC stairmemory: since the error occurred somewhere within an ECC block, theentire ECC block needs to be re-read. In determining the first memorylocation, a required jump time may be taken into account, and anestimated recovery time may be taken into account, i.e. an estimate(based on experience) of the time needed to regain tracking and focus.

Starting from this first memory location, the value of Q is read [step553] from the tracking performance memory 30, and compared with Q_(T)[step 554]. If it appears that Q is above Q_(T), it is not necessary tosearch any further: a jump target location is calculated, being a trackportion 3P(x-n) corresponding to the current tracking performance memorylocation [step 571], and a jump towards this target location 3P(x-n) isexecuted [step 591]. It is noted that 3Px indicates the track portionwhere the error occurs, whereas n indicates the number of track portionsto jump, i.e. the length of the jump.

If, in step 554, it appears that Q is below Q_(T), Q is compared withsaid variable Q_(MAX) [step 561]. If it appears that Q is higher thanQ_(MAX), the value of Q is stored into said variable Q_(MAX), and thecurrent tracking performance memory location is stored in a variableMEMLOC [step 562]. If it appears that the value of Q is not higher thanQ_(MAX), this step 562 is skipped.

Then, the next memory location of the tracking performance memory 30 isconsidered [step 563]. If this memory location is the above-mentionedlast memory location, the analyzing procedure 550 is finished, otherwisethe procedure returns to step 553 [decision step 564].

If, in step 564, it appears that the next memory location is theabove-mentioned last memory location, it means that Q is nowhere aboveQ_(T) (compare FIG. 4C). The jump target location is now determined asbeing a location having the best Q, i.e. corresponding to the memorylocation stored in said variable MEMLOC [step 581], and a jump towardsthis target location is executed [step 591].

After executing the jump, and after having regained tracking and focus,reading is resumed.

As should be clear from the above description, the optical pickup is nowpositioned at a track location before the start address of the ECC blockwhere the error occurred. The distance to this start address is not afixed value; after all, in accordance with the invention, the jumptarget location was a location somewhere before said start address,selected in view of its tracking quality Q. So, it may be that theoptical pickup has to travel some track length until it reaches saidstart address. Therefore, in step 592, it is continuously checkedwhether said start address has been reached; only then, procedurecontinues from step 511.

It should be clear to a person skilled in the art that the presentinvention is not limited to the exemplary embodiments discussed above,but that several variations and modifications are possible within theprotective scope of the invention as defined in the appending claims.

For instance, in the above it is described with reference to FIG. 5 thatthe value of Q is stored into said variable Q_(MAX), and that thecurrent tracking performance memory location is stored into a variableMEMLOC, if Q>Q_(MAX), otherwise this step is skipped. As a result, thevariable MEMLOC will contain the tracking performance memory locationwhere the highest Q occurred for the first time, i.e. corresponding to adisc location closest to the location where the error occurred.Alternatively, however, it is also possible that the current trackingperformance memory location is stored into said variable MEMLOC ifQ≧Q_(MAX): this will result in the variable MEMLOC containing thetracking performance memory location where the highest Q occurred forthe last time, i.e. corresponding to a disc location most remote fromthe location where the error occurred.

Further, in the above explanation, the tracking performance memory isanalyzed starting at a first memory location corresponding to a smalljump and ending at a last memory location corresponding to a large jump.It should be clear that the tracking performance memory may also beanalyzed in the opposite direction.

In the above, the present invention has been explained with reference toblock diagrams, which illustrate functional blocks of the deviceaccording to the present invention. It is to be understood that one ormore of these functional blocks may be implemented in hardware, wherethe function of such functional block is performed by individualhardware components, but it is also possible that one or more of thesefunctional blocks are implemented in software, so that the function ofsuch functional block is performed by one or more program lines of acomputer program or a programmable device such as a microprocessor,microcontroller, etc.

1. Method for reading data from a track (3) of a data carrier (2),comprising the steps of: tracking the track (3) while reading data fromthe track (3), using a tracking performance memory which containsinformation relating to a tracking quality of track portions of the datacarrier (2), to determine, in situations where tracking has to berestored, which track portion to jump to for restoring tracking. 2.Method as claimed in claim 1, further comprising the steps of: duringreading, determining a tracking quality parameter (Q) relating to atrack portion (3Px) of the data carrier from which the data is beingread; storing the tracking quality parameter in the tracking performancememory (30) in relation to the corresponding track portion (3Px). 3.Method as claimed in claim 1 or 2, further comprising the steps of:determining if a jump back to a previously read track portion isnecessary; if a jump back to a previously read track portion appears tobe necessary, consulting said tracking performance memory to determine atarget track portion (3P(x-n)) having a sufficient value of the trackingquality parameter (Q); jumping back (JB) to the target track portion(3P(x-n)) thus determined.
 4. Method according to claim 3, wherein ajump back is performed in case of a data read error.
 5. Method accordingto claim 3, wherein the data read from track is put into a buffer memory(20) at a first data rate, and wherein data is taken out from saidmemory (20) at a second data rate lower than the first data rate, andwherein a jump back is performed in case of a buffer overrun.
 6. Methodaccording to claim 2, wherein the tracking quality parameter (Q) isdetermined on the basis of a tracking error signal.
 7. Method accordingto claim 2, wherein the data carrier is an optical data carrier, andwherein the quality parameter (Q) is determined on the basis of a beamfocus error signal.
 8. Method according to claim 3, further comprisingthe steps of: determining a quality threshold value (Q_(T)); andscanning said tracking performance memory (30) for a memory locationcontaining a tracking quality parameter (Q) having a value above thequality threshold value (Q_(T)); wherein, if at least one trackingquality parameter (Q) having a value above the quality threshold value(Q_(T)) is found, the target track portion (3P(x-n)) is determined asbeing the closest track portion, i.e. having the lowest value of n, forwhich the tracking quality parameter (Q) has a value above the qualitythreshold value (Q_(T)).
 9. Method according to claim 3, furthercomprising the steps of: determining a quality threshold value (Q_(T));scanning said tracking performance memory (30) for a memory locationcontaining a tracking quality parameter (Q) having a value above thequality threshold value (Q_(T)); and determining a maximum value(Q_(MAX)) of the tracking quality parameters (Q) stored in said trackingperformance memory (30); wherein, if not one tracking quality parameter(Q) having a value above the quality threshold value (Q_(T)) is found,the target track portion (3P(x-n)) is determined as being the closesttrack portion, i.e. having the lowest value of n, for which the trackingquality parameter (Q) has the maximum value (Q_(MAX)).
 10. Methodaccording to claim 3, further comprising the steps of: determining aquality threshold value (Q_(T)); scanning said tracking performancememory (30) for a memory location containing a tracking qualityparameter (Q) having a value above the quality threshold value (Q_(T));and determining a maximum value (Q_(MAX)) of the tracking qualityparameters (Q) stored in said tracking performance memory (30); wherein,if not one tracking quality parameter (Q) having a value above thequality threshold value (Q_(T)) is found, the target track portion(3P(x-n)) is determined as being the most remote track portion, i.e.having the highest value of n, for which the tracking quality parameter(Q) has the maximum value (Q_(MAX)).
 11. Method according to claim 3,wherein, if a jump back to a previously read track portion appears to benecessary, an available jump area (42) of said tracking performancememory (30) is determined; and wherein only this available jump area(42) of said tracking performance memory (30) is consulted to determinesaid target track portion.
 12. Method according to claim 11 wherein thedata read from track is inputted into a buffer memory (20) at a firstdata rate, and wherein data is taken out from said buffer memory (20) ata second data rate lower than the first data rate; wherein, if a jumpback to a previously read track portion appears to be necessary, amaximum jump length is determined, depending on the filling level of thesaid buffer memory (20); and wherein a maximum boundary (41) of saidavailable jump area (42) is determined, corresponding to said maximumjump length.
 13. Method according to claim 11, wherein a first memorylocation of said available jump area (42) is determined, correspondingto a minimum jump length, taking into account one or more of thefollowing parameters: required jump time; estimated recovery time; startaddress of data block that was being read when the jump back becamenecessary.
 14. Method according to claim 1, wherein the data carrier (2)is a disc-shaped carrier, which is rotated during reading; wherein thetrack (3) is in the form of a continuous spiral or in the form of aplurality of mutually substantially concentric circles; and wherein thetrack portions (3P) have a track length corresponding to 360°/N, N beingan integer which preferably is larger than 10, and which suitably isequal to
 64. 15. Method of determining a tracking quality of trackportions of a data carrier (2), comprising the steps of: tracking thetrack (3); during tracking, determining a tracking quality parameter (Q)relating to a track portion (3Px) of the data carrier; storing thetracking quality parameter in a tracking performance memory (30) inrelation to the corresponding track portion (3Px).
 16. Method accordingto claim 15, wherein the tracking quality parameter (Q) is determined onthe basis of a tracking error signal.
 17. Method according to claim 15,wherein the data carrier is an optical data carrier, and wherein thequality parameter (Q) is determined on the basis of a beam focus errorsignal.
 18. Data reading apparatus (1), for reading data from a datacarrier (2), adapted to perform the method of claim
 1. 19. Optical discdrive (1) comprising: rotation means (4) for rotating an optical disc(2) having a track (3); an optical pickup (6) for scanning the rotatingoptical disc (2) using a scan beam (7); a radial actuator (8) fordisplacing the scan beam (7) transversely to the track (3); a controller(10) for controlling the actuator (8) to maintain the scan beam on thetrack (3), the controller (10) coupled to receive a read signal (SR)from the optical pickup (6); a data buffer (20) having an input (21)coupled to a data output (11) of the controller (10); a processing unitfor consulting a tracking performance memory which contains informationrelating to a tracking quality of track portions of the optical disc(2), and to determine, in situations where tracking has to be restored,which track portion to jump to for restoring tracking.